a. Field of the Invention
This invention relates to the field of graphics and image processing.
b. Related Art
In the field of imaging and graphics it is often necessary or desirable to be able to mix two or more video signals for display on a video monitor. For example, a window containing a video image from one graphics processor may be inserted onto the video display of another graphics processor, or any other video source such as a camera output or video recording device. Another application would be the creation of video overlays where the video output information from one graphics processor is mixed with the output of a second video source such that certain colors on the first video output are treated as being transparent, i.e. allowing the second source to be visible.
One conventional method of mixing two video signals is by digitally combining the output signals before they are converted into an analog video signal. In order to mix the video outputs from separate systems, the system's processors are synchronized and connected to a common digital pixel data bus. The selection of which video signal to enable onto the pixel bus at any given position on the video display is made by a digital output select control signal. This signal could be produced by a window signal generating circuit or as some function of the image stored in one of the video output generators (for overlays).
The problems with conventional digitally controlled techniques are several fold. First, the hardware required to perform the mixing at the level where the video information is still in its digital form is relatively cumbersome and complex. All of the systems must either be using a commonly structured internal pixel data format or communicate via specially designed interfaces. Also, digital switching techniques are generally slow (compared to the analog video signals) and may degrade system performance unless functioning with a great degree of parallelism (which increases circuit complexity).